When a solid, liquid or gas has to be heated up or cooled down a heat exchanger is used. In a heat exchanger a hot fluid (e.g. hot water, steam or air, etc.) is used to heat a cooler fluid. The two fluids will be separated by some physical barrier, such as, a tube, a wall or a metal plate. The aim of a heat exchanger designer is to make sure that the area of the tube, walls or metal plate is large enough for the required amount of heat to be transferred from the hot fluid to the cold fluid. The performance of a heat exchanger will normally be specified in terms of the inlet and outlet temperatures of one of the two streams entering the exchanger. The amount of heat that has to be transferred between fluid streams is called the heat load. Thus, heat exchangers are devices designed to accomplish efficient heat transfer from one fluid to another and are widely used in engineering processes. Some examples are intercoolers, preheaters, boilers and condensers in power plants. The first law of thermodynamics is generally applicable to a heat exchanger working at steady-state condition and operates in accordance with the following formula:ΣmiΔhi=0where, mi=mass flow of the i-th fluid andΔhi=change of specific enthalpy of the i-th fluid
There are several types of heat exchangers typically available. One type of heat exchanger is the recuperative type, in which fluids exchange heat on either side of a dividing wall. A second type of heat exchanger is the regenerative type, in which hot and cold fluids occupy the same space containing a matrix of material that works alternatively as a sink or source for heat flow. A third type of heat exchanger is the evaporative type, such as cooling tower in which a liquid is cooled evaporatively in the same space as coolant.
The recuperative type of heat exchanger is the most common heat exchanger in practice and the design is usually of one of the following types:
Parallel-Flow Heat Exchanger
A parallel flow heat exchanger usually has a fluid flowing through a pipe and exchanges heat with another fluid through an annulus surrounding the pipe. In a parallel-flow heat exchanger fluids flow in the same direction. If the specific heat capacity of fluids are constant, it can be shown that:dQ/dt=UAΔTwhere,dQ/dt=Rate of heat transfer between two fluidsU=Overall heat transfer coefficientA=Area of the tubeΔT=Logarithmic mean temperature difference defined by:ΔT=(ΔT1−ΔT2)/ln(ΔT1/ΔT2)Cross-Flow Heat Exchanger
In a cross-flow heat exchanger the direction of fluids are perpendicular to each other. The required surface area, Across for this heat exchanger is usually calculated by using tables. It is between the required surface area for counter-flow, Acounter and parallel-flow, Aparallel i.e.Acounter<Across<Aparallel Counter-Flow Heat Exchanger
In a counter-flow heat exchanger a fluid typically flows through a pipe and exchanges heat with another fluid through an annulus surrounding the pipe. In a counter-flow heat exchanger fluids flow in the opposite direction. If the specific heat capacity of fluids are constant, it can be shown that:dQ/dt=UAΔTwhere,dQ/dt=Rate of heat transfer between two fluidsU=Overall heat transfer coefficientA=Area of the tubeΔT=Logarithmic mean temperature difference defined by:ΔT=(ΔT1−ΔT2)/ln(ΔT1/ΔT2)